Graphite refractory molds and method of making same



May 19, 1959 -J. M. WEBB ET AL GRAPHITE REFRACTORY MOLDS AND METHOD OF MAKING SAME Filed Aug. 1, 1956 INVENTORS John M. Webb B Zenon aze as W45 rm ATTORNEYS United States Patent Ofifice 21,886,869 Patented May 19, 1959 GRAPHITE REFRACTORY MOLDS AND METHOD OF MAKING SAME John M. Webb, Chagrin Falls, and Zenon Kazenas, East Cleveland, Ohio Application August 1, 1956, Serial No. 601,535 3 Claims. (Cl. 22-216.5)

This invention relates to refractory molds made of graphite used for precision metal casting and to the method of making the molds. It is particularly related to refractory molds used in the frozen mercury casting process.

In the frozen mercury process, a frozen mercury pattern is formed by dipping the pattern into two separate slurries of refractory materials suspended in non-aqueous volatile liquid carrier so as to coat the pattern with first and second mold layers. The mercury is liquefied and drained from the mold layers to form a hard thin-walled mold which is thereafter baked to form a hard thinwalled mold suitable for casting of molten metals. The refractory compositions are generally made of finely divided refractory particles such as zirconium silicate which are slurried in a low boiling dispersant such as a Freon. The composition also contains an organic resin as a low temperature binder to hold the particles together on evaporation of liquid from the slurry and a high temperature binder such as ammonium phosphate which becomes effective as a binder at elevated temperatures.

The refractory materials generally used are oxides or silicates such as alumina, silica, zirconium silicate and aluminum silicate which may not only be relatively expensive, but often may react with the metals cast into molds made with such refractory materials. This reaction causes a rough spongy condition on the surface of the casting and usually occlusion of gases Within the casting.

High temperature binders must be used to bind the refractory materials together upon baking the green mold. A preferred high temperature binder is mono basic ammonium phosphate since it sets up rapidly and at relatively low temperatures but its use is generally restricted to the outer layers because of the tendency of phosphorous to contaminate the casting by migration to the inner casting surface. Even when the ammonium phosphate is used in the outer layers, there is danger of phosphorous contamination.

It is very difficult to cast high melting temperature reactive metals such as titanium and its alloys into thin shell molds due to the reaction between the mold materials and the metal.

Another disadvantage of the process previously described is the fact that at least two different slurries must be prepared and maintained at a low temperature of about --60 P. which is expensive and time consuming.

It is an object of the present invention to provide a refractory mold which can be used for the precision casting of titanium metal.

It is an object to provide an inexpensive mold for the precision casting of low melting metals such as aluminum and bronzes.

One object of the present invention is to provide a mold in which no high temperature binder is used so as to eliminate the danger of contamination of the cast metal.

Another object of the present invention is to provide a method of making molds in which only a single slurry need be prepared and maintained at low temperature.

Other objects, uses and advantages of the present invention will become apparent to those skilled in the art from the following description and claims and from the drawings in which:

Figure 1 is a perspective view of a hook used on the tail of aircraft to be made in accordance with this invention;

Figure 2 is a fragmentary elevational view of a frozen mercury pattern for the hook with parts broken away and shown in cross section;

Figure 3 is a fragmentary elevational view of the frozen mercury pattern with a refractory coating formed thereon and with parts broken away and shown in cross section, and

Figure 4 is an elevational view of the mold used for casting the hook with parts broken away and shown in cross section.

In accordance with the present invention, the above objects are accomplished by making the mold of a composition which is predominantly graphite and a low temperature binder and which requires no high temperature binder. I have found that the mold is non-reactive with high melting metals such as titanium, and also low melting metals such as aluminum, aluminum bronzes, copperzinc alloys and magnesium-manganese alloys. A high melting pitch, such as coal tar pitch, is preferably used in the composition to impart greater strength to the mold wall and bind the graphite particles together after the low temperature binder has been removed by baking.

The preparation of the mold and its use may be conveniently explained by reference to the drawings where the formation of a mold for forming a product such as the hook used on the tail of airplanes is illustrated. Hook 5 has a generally flat rectangular base portion 6 and an eye portion 7 having a generally circular opening 8.

Figure 2 shows a frozen mercury pattern 11 of hook 5 with a sprue 15 of frozen mercury attached thereto by means of linking sections 16 of frozen mercury which will provide passageways to the interior of the mold as hereafter described with the aid of the hollow frozen mercury pattern 11. A metal hook 17 is frozen in the sprue portion 15 for handling the pattern 11 during the mold forming operation.

In the present invention, the mold layers are preferably formed on a frozen mercury pattern since mercury has a low coefiicient of expansion which is close to zero at its melting point. While pure mercury is preferred, alloys of mercury or mercury with impurities contained therein may also be used satisfactorily. A method of producing castings such as the lost-wax method may be used if the desired molds are relatively thick. Other solid materials such as low-melting solders may be used for the pattern providing the material can be remove without damaging the mold.

In accordance with the present invention, a frozen mercury pattern 11 is prepared by shaping the mercury in a steel or metal alloy mold which is not attacked by mercury. The frozen mercury pattern 11 is dipped into a slurry of predominantly graphite particles but preferably containing a small amount of a binder such as pitch to form a thin shell mold 20 over the exposed exterior surfaces of the pattern. The mold walls 21 may be built up to the desired thickness by dipping the pattern in the slurry, allowing the coating to dry and then repeating the dipping operation. In some instances to avoid any contamination from a binder in contact with the casting it is desirable to have the pattern first dipped in a slurry having only graphite as the solid and thereafter to dip the once or twice coated mold into a slurry of graphite and binder. The mercury is subsequently liquefied and drained from the mold layers. to form the mold 20.

The total thickness of the mold walls is generally about A to inch although the walls may be thinner or thicker for certain applications. Generally, the walls should be thin so that they can yield to the expanding and contracting forces brought about by the contact with and shrinking of the hot metal upon solidifying.

The green mold 20 is held together by suitable low temperature binders in the slurry until the mold is fired at a temperature of about 1000 F. to 2300 F. so that it forms a hard thin-walled mold.

The principal refractory ingredient of the mold walls is, as before stated, graphite, which is a shiny soft naturally occurring substance of plate like structure having a specific gravity of 2.2 and 2.3 and is classified in the hexagonal crystal system. Natural graphite consists largely of carbon, generally 50-97 percent, and it may have impurities such as mica fragments therein. A more pure form of graphite, which is made artificially, may also be used. When the amounts by weight of graphite in the solid composition of the slurry used to form the mold wall is less than 50 percent and the remainder is binder removed by burning at high temperatures, the mold may be too porous for some applications, although amounts as low as 40 percent for some applications, particularly when the residue from the large quantities of a high melting pitch and a low temperature binder used is unobjectionable to the metal of the casting. Generally about 70 or 80 to 90 percent of graphite is used in the mold forming slurry solids. The particle size of the graphite is preferably from 100 to 400 mesh depending upon the grain size desired in the finished casting. It may be desirable in solid mold designs to form the mold layer from an inner casting portion containing finely divided graphite particles and an outer portion containing coarser particles, say of a size 60 to 100 mesh, which larger size imparts greater strength to the mold wall.

Low temperature binders such as organic resins of which polyvinyl acetates and phenolformaldehyde resols and novolacs are generally used, hold the green mold together at low temperatures. The low temperature binders preferably are used in percentages of 1 to 5 percent by weight although most of the benefits of the invention may be obtained with as low as 0.25% up to as high as Usually the use of over 10% of the resins results in their incomplete removal from the mold wall by burning. Examples of such organic resins are meltable or liquid phenol formaldehyde resins, alkyd or polyester resins, hydrocarbon resins such as coumarin indene resins, polyvinyl acetate cellulose esters, etc. When pitch is used, the amount of low temperature binder used is generally kept low, about 1 to 3 percent, which amounts are preferred to facilitate the covering of the frozen mercury pattern.

A high melting pitch such as coal tar pitch may be used in the composition in amounts preferably of to 45 percent although good results may be obtained with as little as 1 to 2 percent and with as high as 50%. The pitch preferably is used in a finely divided form or as a solute in a solvent so as to produce a smooth uniform finished mold.

When pitch is used, the green molds are fired preferably in a reducing atmosphere so that the heat causes the pitch to burn in an atmosphere with a limited supply of oxygen. The pitch decomposes and burns to form a large amount of amorphous carbon or soot. A large volume of gas is also generated which carries and distributes the carbon to the raphite particles. The amorphous carbon particles deposit on the surface of the graphite particles and in some way interact with them to bond the graphite particles together.

A high melting pitch is preferably used to cast high melting temperature metals such as titanium and preferably is used when the molten metals have a casting temperature substantially over 2000" F. Mold slurry compositions of graphite and a low temperature binder are suitable for metals having a melting point not substantially greater than 2000 F. such as aluminum and bronzes, although best results are obtained when at least 15% of the composition is pitch.

A frozen mercury pattern of an eye-hook for the tail of an airplane similar to that shown in Figure 2 of the drawings was dipped in a slurry shown below a sufficient number of times to form a refractory layer 4 inch thick.

MOLD LAYER SLURRY The layer was formed by building up a plurality of coatings on the exterior surface of the pattern by allowing each coating to dry before starting the next dipping and drying operation.

Thereafter the mercury was liquefied and drained from the mold layer so that a thin-walled mold was formed. The mold was fired at 700 F. in an inert atmosphere of helium and argon.

Thereafter, the mold was backed with sand in a sup porting flask and both the mold and investing material heated. Molten aluminum was cast into the mold at about 1600 F. to form an excellent casting. The surface of the casting was smooth and no defects were observed thereon.

Example II A casting similar to that of Example I was made of aluminum bronze copper, 10% aluminum) in a manner similar to that of Example I except that a different slurry was used to form the mold layer as follows:

Amounts Percent Ingredients (grams) Weight Graphite 970 97 Ethyl Cellulose- 20 2 Polyvinyl Acetate. 10 1 Total Solids 1, 000 100 Liquid Carrier: Monochlorodifiuoromethane 800 The resultant casting, which was poured at 2200 F., was excellent.

Example III A casting of aluminum was made as described in Example I except that the slurry of Example I was sub stituted for by a slurry composed as follows:

The resultant casting was satisfactory.

Example IV A casting of aluminum-bronze was prepared by a mold formed from a slurry identical to that of Example II. After the mold was fired, the mold was impregnated by 5 pouring a liquid comprising Epon 1062 (an epoxy type resin sold by the Shell Chemical Co.) and diethylenetriamine as a crosslinking agent as into the mold cavity. The impregnating composition is shown below:

Ingredients: Amount (grams) Epon 1062 (a reaction product of epichlorohydrin and Bisphenl-A) 100 Diethylenetriamine 15 The liquid was allowed to wet the inner mold surfaces for several minutes, after which, the excess resin mixture was poured out. The mold did not soften during impregnation and after 30 minutes a hard mold was obtained with good green strength. The mold was thereafter cast into to form a good aluminum bronze casting.

Example V A mold was prepared as described in Example I by use of the following slurry:

The green mold was covered with a calcined coke and fired in a furnace at 2000 F. without the use of an inert atmosphere.

After the mold was fired, it was invested with sand and heated to 1800 F. Molten titanium, at a temperature of about 3200 F. was cast into the mold in a vacuum chamber having a helium atmosphere. An excellent casting was obtained with no evidence of a spongy surface or entrapped gas within the casting.

The green molds should be fired under non-oxidizing conditions so as to prevent carbon from being burned out of the mold. They may be fired in a non-oxidizing atmosphere of inert gas such as nitrogen, helium or argon, or in a vacuum of less than mm. of mercury and preferably less than 1 mm. of mercury pressure. The surfaces of the mold may also be covered with a calcined carbon such as calcined petroleum coke which acts initially as a covering or blanket for the mold so that it is fired in an atmosphere of limited oxygen. The calcined carbon probably forms some carbon monoxide as it burns which tends to provide a reducing atmosphere around the mold itself.

The polyvinyl acetate and ethyl cellulose, which are the prepared low temperature binders, may be substituted for in whole or part by other low temperature binders such as epon resins, polymerized normal-butyl methacrylate, and even rubbery copolymers such as copolymers of 30-70% acrylonitrile and 70-30% butadiene.

The monochlorodifluoromethane used in the previous examples may be substituted in whole or part by other volatile dispersants which are liquid at the temperature the mercury is solid and which have high enough boiling points to evaporate when the mercury becomes liquid. Other liquid carriers suitable for this invention are liquefied methyl chloride, difluorodichloromethane, trichloromonofluoromethane, or a mixture of two or more.

Other liquid dispersants having higher boiling points may be used when the casting process is the lost-wax method or when the solid material is a low melting solder such as Woods metal. Examples of dispersants that might be used in the above methods are water, carbon tetrachloride, alcohol and benzene.

The graphite may be replaced at least in part by carbon black or activated charcoal in percentages not substantially greater than 25% by weight, although due to the increased surface area of the above named carbon particles, larger amounts of low temperature hinder and/or pitch are generally used.

If desired, the mold layers of the present invention may be hacked and supported by outer refractory portions formed in a similar manner from slurries comprising a zirconium silicate, a low temperature binder and a high temperature binder such as mono basic ammonium phosphate.

The coal tar pitch of Example V may be substituted for by other volatilizing high carbon producing hydrocarbon compounds having a melting point of from about 325 F. to 500 F. or even up to 750 P. such as pine pitch, crude naphthalene rosin pitch or sea coal pitch.

The impregnating composition of Example IV may be substituted by other impregnating compositions having other organic resins named herein as low temperature binders, such as polyvinyl acetate and polybutyl methacrylate.

It is well understood that, in accordance with the provisions of the patent statutes, variations and modifications of the specific invention may be made without changing the spirit thereof.

What we claim is:

1. A method of casting chemically active metals and alloys comprising the steps of preparing a mold from a solid pattern by dipping the pattern into a slurry consisting of a refractory composition of graphite particles of 35 to 400 mesh, 10 to 50 parts by weight of a high melting pitch and .25 to 10 parts by weight of a low temperature binder in a carrier to build up a plurality of thin layers to form a thin-walled mold, melting out the pattern, heating the mold under non-oxidizing conditions to a temperature high enough to decompose the binder and pitch to amorphous carbon, investing the mold, preheating the invested mold, and pouring the molten metal into it.

2. A method of casting titanium and titanium alloys comprising the steps of preparing a mold from a solid pattern by dipping the pattern into a slurry consisting of a refractory composition of graphite particles of 35 to 400 mesh, 10 to 50 parts by weight of a high melting pitch and .25 to 10 parts by weight of a low temperature binder in a carrier to build up a plurality of thin layers to form a thin-walled mold, melting out the pattern, heating the mold under non-oxidizing conditions to a temperature high enough to decompose the binder and pitch to amorphous carbon, investing the mold, preheating the invested mold, and pouring the molten metal into it.

3. A method of manufacturing a thin-walled refractory mold from a solid pattern comprising the steps of dipping the pattern into a slurry consisting of a refractory composition of graphite particles of 35 to 400 mesh, 10 to 50 parts by weight of a high melting pitch and .25 to 10 parts by weight of a low temperature binder in a carrier to build up an inner wall portion, dipping the pattern into additional refractory composition slurries as necessary to form a thin-walled mold, melting out the pattern, heating the mold under non-oxidizing conditions to a temperature high enough to decompose the binder and pitch to amorphous carbon, cooling the mold, and impregnating the mold with a low temperature binder in liquid form for sufiicient time to cause the binder to penetrate at least the inner and outer portions of the mold.

References Cited in the file of this patent UNITED STATES PATENTS 2,521,614 Valyi Sept. 5, 1950 2,668,774 Heyl Feb. 9, 1954 2,749,586 Kohl et a1. June 12, 1956 FOREIGN PATENTS 725,456 Great Britain Mar 2, 1955 OTHER REFERENCES Steel, Dec. 28, 1953, page 88. 

1. A METHOD OF CASTING CHEMICALLY ACTIVE METALS AND ALLOYS COMPRISING THE STEPS OF PREPARING A MOLD FROM A SOLID PATTERN BY DIPPING THE PATTERN INTO A SLURRY CON SISTING OF A REFRACTORY COMPOSITION OF GRAPHITE PARTICLES OF 35 TO 400 MESH, 10 TO 50 PARTS BY WEIGHT OF A HIGH MELTING PITCH AND .25 TO 10 PARTS BY WEIGHT OF A LOW TEMPERATURE BINDER IN A CARRIER TO BUILD UP A PLURALITY OF THIN LAYERS TO FORM A THIN-WALLED MOLD, MELTING OUT THE PATTERN, HEATING THE MOLD UNDER NON-OXIDIZING CONDITIONS 